Changes in the processes controlling sleep that accompany aging result in widespread complaints of sleep disturbance in otherwise healthy older individuals. It is likely that age-related changes in many physiological systems interact to produce these sleep problems. One such system is the hypothalamic-pituitary-gonadal (HPG) axis. Gonadal steroids exhibit significant declines with age, resulting in altered negative feedback to the hypothalamus, and unregulated production of luteinizing hormone (LH) and follicle-stimulating hormone (FSH). Recent evidence demonstrates that these alterations continue and worsen with increasing age. That changing levels of gonadal hormones affect sleep quality in aging is supported by both anecdotal and empirical evidence. For example, up to two-thirds of perimenopausal women complain of sleep problems. Further, higher LH levels are associated with awakenings from sleep, and hormone replacement therapy improves objectively-measured sleep. Although investigations of sex hormones and sleep in men are rare, lower levels of testosterone in aging men have been associated with poorer sleep quality. Indeed, the few studies that have assessed sleep complaints in the context of age-associated changes in HPG functioning strongly suggest that further investigation is needed. In addition, while changes in the circadian timing system are widely believed to contribute to sleep problems in older individuals, little is known about whether circadian patterns of sex hormone release are modified as a function of aging. The proposed research plan combines a chronobiological approach to the study of changes in the HPG axis in aging with an investigation of whether sex hormone levels are associated with age-related sleep problems. One study will assess whether there are circadian rhythms in sex hormones independent of sleep, and whether patterns of gonadal hormone release are altered with aging. The second study will examine whether the balance of sex steroids and gonadotropins differs between older individuals without sleep complaints and those with sleep maintenance insomnia. The combination of findings from these studies will clarify our understanding of how sex hormones, sleep, and circadian rhythmicity interact in aging, providing an avenue for investigating hormone replacement therapies as treatments for sleep problems in older individuals.